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What makes a certain makers heat treat better than anothers?

I don't know where to post this, but this seems the right place. I have heard several people say "_____'s heat treat on AEB-L is the best there is." and "____'s heat treat has gotten a lot better since he first started." So I am wondering what makes certain heat treatments better than others. I guess what I am asking is if I hand you two pieces of 52100 that have been heat treated to 61hrc by 2 different makers and another that has been heat treated to 61hrc by a professional heat treatment company what are the differences and why is one better than the other even though they are the same hrc?

I will expose whatever combination of knowledge and ignorance I have on this topic to hope I dispell some erroneous ideas I have...

I think that for a given HRC, the differences are about toughness vs chippiness. VG10 seems to be a good example of this....in the wrong hands (Shun seems to be blamed for this a lot) it's very chippy, but in good hands, it can be a great steel.

Also, I think limiting the discussion to a given HRC might be useful for discussion purposes, but different makers will choose different HRC's as part of the overall balance they are trying to achieve.

There is a window of time when transformation from austenite to martensite happens (hardening), and that window is quite short (2-3 minutes). In other words, if one works with small batches, one can get blanks quicker from one step of HT to another and that can result in more transformation from austenites to martesites (hardness, more uniform grain structure, etc). Resilient burr, wire edge, often are the symptoms of retained austenites (those that didn't convert to martensites).

If one optimizes steps in HT process, picks the right temperatures from which to quench and at which to temper, one should get a well rounded knife that would sharpen easily, take a keen edge and hold it without rolling or chipping, and retain sharpness for a long time. In case of 52100, if you manage to refine grain, quench at the right temperature, temper at a right temperature, cryo treatment, of course, you get a very fine structure of the steel that feature all of the above qualities and responds very well to stropping on diamond. This will prolong time between sharpening sessions for months - a good thing for pro users, I was told.

"All beauty that has no foundation in use, soon grows distasteful and needs continuous replacement with something new." The Shakers' saying.

Also, I think limiting the discussion to a given HRC might be useful for discussion purposes, but different makers will choose different HRC's as part of the overall balance they are trying to achieve.

I was limiting the discussion. I guess I was asking about the differences between this sample of 52100 at 61hrc and another sample at the same hardness made by a different maker.

Originally Posted by Marko Tsourkan

Custom heat treatment offers some advantages over commercial.

There is a window of time when transformation from austenite to martensite happens (hardening), and that window is quite short (2-3 minutes). In other words, if one works with small batches, one can get blanks quicker from one step of HT to another and that can result in more transformation from austenites to martesites (hardness, more uniform grain structure, etc). Resilient burr, wire edge, often are the symptoms of retained austenites (those that didn't convert to martensites).

If one optimizes steps in HT process, picks the right temperatures from which to quench and at which to temper, one should get a well rounded knife that would sharpen easily, take a keen edge and hold it without rolling or chipping, and retain sharpness for a long time. In case of 52100, if you manage to refine grain, quench at the right temperature, temper at a right temperature, cryo treatment, of course, you get a very fine structure of the steel that feature all of the above qualities and responds very well to stropping on diamond. This will prolong time between sharpening sessions for months - a good thing for pro users, I was told.

That was exactly what I was looking for. So basically two identical pieces of steel that are heat treated to the same hrc will still have different performance based on the amount of time they spend in the "oven"....?.

Also if there is a 2-3min. window where there are both austenites and martensites and austenites are bad why not wait 4 minutes so you only get martensites?

I was limiting the discussion. I guess I was asking about the differences between this sample of 52100 at 61hrc and another sample at the same hardness made by a different maker.

That was exactly what I was looking for. So basically two identical pieces of steel that are heat treated to the same hrc will still have different performance based on the amount of time they spend in the "oven"....?.

Also if there is a 2-3min. window where there are both austenites and martensites and austenites are bad why not wait 4 minutes so you only get martensites?

There is a number of factors. For every steel there is a range of temperatures it could be heat treated (austenitizing and tempering), and the outcome will depend on those temperatures, time soaked at that temperature, speed of cooling, cryo treatment and other steps.

I am just going to run a two hypothetical cases scenario. To simplify things, I will take cryo out of the equation and in both cases steel is soaked during austenitising and tempering same amount of time.

Scenario #1
If you heat treat at close to the lower temperature in the range, you get smaller grain size (more toughness, easier to sharpen) but wear resistance will suffer, as at the lower temp you get less of the components dissolved into a steel matrix. Nonetheless, say you get 64RC as quenched. You temper at a certain temperature, 2 sessions 1 hour each, and the end result is 61RC. Totally doable.

Scenario #2
If you heat treat at close to the upper temperature in the range, you get a larger grain size (less toughness, more wear resistance). Say, as quenched you get 66RC. You raise your tempering temperature, and do 2 sessions of tempering and the end result is 61RC.

First one will perform very differently than the second one - it will be easier to sharpen, and will have very stable edge. Second will have more wear resistance, but might be prone to chipping.

What I described is the window after the quench, while the transformation is still occurring. Certain steps in custom heat treatment allow to reduce retained austenites to the minimum (if not zero). You will know that by increased hardness of your heat treated blanks and ease of sharpening - burr will raise very evenly and will be easy to remove. There is a reason why commercial HT blades are in the 60-61RC range, as when you heat treat a large batch there is a time lag between processes, and that short window of opportunity is missed. For instance, cryo treatment the next day makes no difference on the steel whatsoever.

Hope this makes sense.

A good place to read about this stuff is Devin's subforum and website.
M

"All beauty that has no foundation in use, soon grows distasteful and needs continuous replacement with something new." The Shakers' saying.